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bpf,x86: Respect X86_FEATURE_RETPOLINE* 

Current BPF codegen doesn't respect X86_FEATURE_RETPOLINE* flags and
unconditionally emits a thunk call, this is sub-optimal and doesn't
match the regular, compiler generated, code.

Update the i386 JIT to emit code equal to what the compiler emits for
the regular kernel text (IOW. a plain THUNK call).

Update the x86_64 JIT to emit code similar to the result of compiler
and kernel rewrites as according to X86_FEATURE_RETPOLINE* flags.
Inlining RETPOLINE_AMD (lfence; jmp *%reg) and !RETPOLINE (jmp *%reg),
while doing a THUNK call for RETPOLINE.

This removes the hard-coded retpoline thunks and shrinks the generated
code. Leaving a single retpoline thunk definition in the kernel.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.614772675@infradead.org
This commit is contained in:
Peter Zijlstra 2021-10-26 14:01:48 +02:00
parent dceba0817c
commit 87c87ecd00
3 changed files with 41 additions and 86 deletions
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59
arch/x86/include/asm/nospec-branch.h
View File

@ -316,63 +316,4 @@ static inline void mds_idle_clear_cpu_buffers(void)
#endif /* __ASSEMBLY__ */
/*
* Below is used in the eBPF JIT compiler and emits the byte sequence
* for the following assembly:
*
* With retpolines configured:
*
* callq do_rop
* spec_trap:
* pause
* lfence
* jmp spec_trap
* do_rop:
* mov %rcx,(%rsp) for x86_64
* mov %edx,(%esp) for x86_32
* retq
*
* Without retpolines configured:
*
* jmp *%rcx for x86_64
* jmp *%edx for x86_32
*/
#ifdef CONFIG_RETPOLINE
# ifdef CONFIG_X86_64
# define RETPOLINE_RCX_BPF_JIT_SIZE 17
# define RETPOLINE_RCX_BPF_JIT() \
do { \
EMIT1_off32(0xE8, 7); /* callq do_rop */ \
/* spec_trap: */ \
EMIT2(0xF3, 0x90); /* pause */ \
EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
EMIT2(0xEB, 0xF9); /* jmp spec_trap */ \
/* do_rop: */ \
EMIT4(0x48, 0x89, 0x0C, 0x24); /* mov %rcx,(%rsp) */ \
EMIT1(0xC3); /* retq */ \
} while (0)
# else /* !CONFIG_X86_64 */
# define RETPOLINE_EDX_BPF_JIT() \
do { \
EMIT1_off32(0xE8, 7); /* call do_rop */ \
/* spec_trap: */ \
EMIT2(0xF3, 0x90); /* pause */ \
EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
EMIT2(0xEB, 0xF9); /* jmp spec_trap */ \
/* do_rop: */ \
EMIT3(0x89, 0x14, 0x24); /* mov %edx,(%esp) */ \
EMIT1(0xC3); /* ret */ \
} while (0)
# endif
#else /* !CONFIG_RETPOLINE */
# ifdef CONFIG_X86_64
# define RETPOLINE_RCX_BPF_JIT_SIZE 2
# define RETPOLINE_RCX_BPF_JIT() \
EMIT2(0xFF, 0xE1); /* jmp *%rcx */
# else /* !CONFIG_X86_64 */
# define RETPOLINE_EDX_BPF_JIT() \
EMIT2(0xFF, 0xE2) /* jmp *%edx */
# endif
#endif
#endif /* _ASM_X86_NOSPEC_BRANCH_H_ */

46
arch/x86/net/bpf_jit_comp.c
View File

@ -387,6 +387,25 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
return __bpf_arch_text_poke(ip, t, old_addr, new_addr, true);
}
#define EMIT_LFENCE() EMIT3(0x0F, 0xAE, 0xE8)
static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
{
u8 *prog = *pprog;
#ifdef CONFIG_RETPOLINE
if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
EMIT_LFENCE();
EMIT2(0xFF, 0xE0 + reg);
} else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
} else
#endif
EMIT2(0xFF, 0xE0 + reg);
*pprog = prog;
}
/*
* Generate the following code:
*
@ -468,7 +487,7 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
* rdi == ctx (1st arg)
* rcx == prog->bpf_func + X86_TAIL_CALL_OFFSET
*/
RETPOLINE_RCX_BPF_JIT();
emit_indirect_jump(&prog, 1 /* rcx */, ip + (prog - start));
/* out: */
ctx->tail_call_indirect_label = prog - start;
@ -1179,8 +1198,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
/* speculation barrier */
case BPF_ST | BPF_NOSPEC:
if (boot_cpu_has(X86_FEATURE_XMM2))
/* Emit 'lfence' */
EMIT3(0x0F, 0xAE, 0xE8);
EMIT_LFENCE();
break;
/* ST: *(u8*)(dst_reg + off) = imm */
@ -2084,24 +2102,6 @@ cleanup:
return ret;
}
static int emit_fallback_jump(u8 **pprog)
{
u8 *prog = *pprog;
int err = 0;
#ifdef CONFIG_RETPOLINE
/* Note that this assumes the the compiler uses external
* thunks for indirect calls. Both clang and GCC use the same
* naming convention for external thunks.
*/
err = emit_jump(&prog, __x86_indirect_thunk_rdx, prog);
#else
EMIT2(0xFF, 0xE2); /* jmp rdx */
#endif
*pprog = prog;
return err;
}
static int emit_bpf_dispatcher(u8 **pprog, int a, int b, s64 *progs)
{
u8 *jg_reloc, *prog = *pprog;
@ -2123,9 +2123,7 @@ static int emit_bpf_dispatcher(u8 **pprog, int a, int b, s64 *progs)
if (err)
return err;
err = emit_fallback_jump(&prog); /* jmp thunk/indirect */
if (err)
return err;
emit_indirect_jump(&prog, 2 /* rdx */, prog);
*pprog = prog;
return 0;

22
arch/x86/net/bpf_jit_comp32.c
View File

@ -15,6 +15,7 @@
#include <asm/cacheflush.h>
#include <asm/set_memory.h>
#include <asm/nospec-branch.h>
#include <asm/asm-prototypes.h>
#include <linux/bpf.h>
/*
@ -1267,6 +1268,21 @@ static void emit_epilogue(u8 **pprog, u32 stack_depth)
*pprog = prog;
}
static int emit_jmp_edx(u8 **pprog, u8 *ip)
{
u8 *prog = *pprog;
int cnt = 0;
#ifdef CONFIG_RETPOLINE
EMIT1_off32(0xE9, (u8 *)__x86_indirect_thunk_edx - (ip + 5));
#else
EMIT2(0xFF, 0xE2);
#endif
*pprog = prog;
return cnt;
}
/*
* Generate the following code:
* ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) ...
@ -1280,7 +1296,7 @@ static void emit_epilogue(u8 **pprog, u32 stack_depth)
* goto *(prog->bpf_func + prologue_size);
* out:
*/
static void emit_bpf_tail_call(u8 **pprog)
static void emit_bpf_tail_call(u8 **pprog, u8 *ip)
{
u8 *prog = *pprog;
int cnt = 0;
@ -1362,7 +1378,7 @@ static void emit_bpf_tail_call(u8 **pprog)
* eax == ctx (1st arg)
* edx == prog->bpf_func + prologue_size
*/
RETPOLINE_EDX_BPF_JIT();
cnt += emit_jmp_edx(&prog, ip + cnt);
if (jmp_label1 == -1)
jmp_label1 = cnt;
@ -2122,7 +2138,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
break;
}
case BPF_JMP | BPF_TAIL_CALL:
emit_bpf_tail_call(&prog);
emit_bpf_tail_call(&prog, image + addrs[i - 1]);
break;
/* cond jump */